1. Field of Invention
This invention relates to tools and methods used to diagnose and repair faults in a grouping of series-connected lamps, typically used for decorative lighting during various holidays.
2. Prior Art
Strings of lights are widely used for decoration during major holidays such as Christmas, Halloween, the Fourth of July and others. The dominant configurations of these strings use a series-connected group of low voltage incandescent lamps or light emitting diodes (LED). The number of lamps in a string varies, but is typically 25-50 in order to match the line voltage in the United States and other countries.
Due to the series configuration, such strings have the unfortunate characteristic that a single lamp failure will cause all others to extinguish, thus making the task of fault diagnosis difficult. The conventional method for repairing such strings is to sequentially unplug each lamp, if indeed they are replaceable, and substitute a known good lamp in its place. For a 50 lamp string, this can require up to 50 separate operations. If more than one lamp has failed, this process can require over 1000 separate tests.
Many patents have attempted to address this problem. U.S. Pat. No. 3,725,780 (1973) to Olin shows a simple battery powered lamp tester which reduces the maximum number of tests to 50, even for multiple failures, but the approach fails to account for faults in the lamp socket or wiring. It further requires that the lamps be removable and is problematic for LED lamps due to polarity and reverse breakdown voltage issues. It further uses batteries which, if depleted, yield a false indication of a failure.
Incandescent lamps with various shunting devices have been proposed which activate in the event of an open filament. See U.S. Pat. No. 7,279,809 (2007), U.S. Pat. No. 7,166,968 (2007), U.S. Pat. No. 6,323,597 (2001) all to Janning. The most common commercial arrangement involves several turns of thinly-insulated wire wrapped around the support leads for the lamp filament. The wire is designed to short the lamp if voltage substantially greater than its design value is applied. Theoretically this happens if the filament opens and allows the remaining lamps to remain illuminated, thus highlighting the single failed lamp. In practice this arrangement is only partially effective and strings of such lamps experience frequent failures of the shunting mechanism. In addition to not being highly effective, any shunting device or structure adds to the cost of each and every lamp and/or socket in the string.
U.S. Pat. No. 6,480,001 (2002) to Frederick discloses a high voltage pulse device which, when attached to a faulty incandescent string, attempts to activate an otherwise inoperative shunt device. This scheme is only partially effective due to the variation in shunt characteristics, and is generally ineffective when more than one lamp is faulty. Moreover, this scheme is not effective with, and potentially injurious to, LED strings. Lamp failures not caused by a shunted open filament are not addressed.
U.S. Pat. No. 5,047,721 (1991) to Farley and U.S. Pat. No. 6,556,018 (2003) to Benton attempt to exploit the elevated electric field in the vicinity of an open filament lamp when its parent string is energized. The presence of such an increased field theoretically shows the location of the faulty lamp. This method again is only partly effective due to the variation in lamp configuration and operator diligence and is ineffective when more than one lamp has failed. Moreover, it does not work for LED strings which use direct current.
Various probing methods have been proposed which require lamps to be removed e.g. U.S. Pat. No. 5,319,312 (1994) to Segilia, or which use external batteries and pins to contact the internals of a lamp socket e.g. U.S. Pat. No. 5,179,339 (1993) to Volk. The socket probing method fails where the connecting wires are molded into the socket base, thus preventing the connection to lamp socket internal contacts. Both approaches fail to address LED strings; in Segilia due to the lamp's high resistance under low applied voltages and in Volk, due to potentially injurious high reverse voltage if connected incorrectly. Furthermore, removing the lamps for testing (Segilia) often introduces additional failures thus extending the process and frustrating the user.
U.S. Pat. No. 5,604,436 (1997) to Henritzy et al. displays a probe apparatus that is powered from the 110 volt mains, but features an energized probe member which can be inadvertently touched by the operator despite its nominal guarding structure. It also requires that the internal contacts of the socket be accessible from the bottom, and is thus inoperative if wires are molded into socket bases.
US patent application US2012/0206146 (2012) to Avenel shows a method to indicate that at least one LED in a series-connected string is faulty, but fails to find which one is defective and requires a detailed measurement of voltages at different points along the string. Such measurements involve expensive, technical equipment far beyond the abilities and budgets of most consumers and are very difficult to conduct on typical holiday lamp strings.